A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
It is desirable to provide a lithographic apparatus that allows one to accurately control the dose of radiation received by each point on the target portion of the substrate. The dose of radiation received by a part of the target area of the substrate may be defined as the amount of energy per unit area which is received by that part. Accurate control of the dose of radiation in turn allows variation of the critical dimension of features formed on the substrate to be controlled.
It is an object of the present invention to provide a lithographic apparatus and a device manufacturing method that at least partially addresses one or more of the problems of the prior art, whether identified herein or elsewhere.